1. Field of the Invention
The present invention relates to a semiconductor device.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the present invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. Further, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a storage device, a method for driving any of them, and a method for manufacturing any of them.
In this specification and the like, a semiconductor device means any device that can function by utilizing semiconductor characteristics. A transistor and a semiconductor circuit are embodiments of semiconductor devices. A storage device, a display device, or an electronic device may include a semiconductor device.
2. Description of the Related Art
Storage devices using semiconductor elements are broadly classified into two categories: a volatile storage device that loses stored data when power supply stops, and a non-volatile storage device that holds stored data even when power supply stops.
A typical example of a volatile storage device is a dynamic random access memory (DRAM). A DRAM stores data in such a manner that a transistor included in a storage element is selected and charge is accumulated in a capacitor.
When data is read from a DRAM, charge in a capacitor is lost according to the above principle; thus, another writing operation is needed whenever data is read. Moreover, a transistor in a storage element that is in an off state has a leakage current (off-state current) between a source and a drain or the like and thus charge flows into or out of a capacitor, which makes a data holding period short. Accordingly, writing operation (refresh operation) needs to be performed at predetermined intervals, resulting in relatively high power consumption. Furthermore, since stored data is lost when power supply stops, an additional storage device using a magnetic material or an optical material is needed in order to hold the data for a long time.
Another example of a volatile storage device is a static random access memory (SRAM). An SRAM holds stored data by using a circuit such as a flip-flop and thus does not need refresh operation, which is an advantage over a DRAM. An SRAM, however, has a problem that cost per storage capacity is increased because the occupation area of a flip-flop is large. Furthermore, as in a DRAM, stored data in an SRAM is lost when power supply stops.
A typical example of a nonvolatile storage device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region of a transistor and stores data by holding charge in the floating gate. Therefore, a flash memory has advantages in that a data holding period is extremely long (almost permanent) and refresh operation needed in a volatile storage device is not needed (e.g., see Patent Document 1).
However, a gate insulating layer included in a storage element deteriorates owing to a tunneling current generated in writing, so that the storage element stops its function after a predetermined number of times of writing. In order to reduce effects of this problem, a method in which the number of times of writing for storage elements is equalized is employed, for example. However, a complicated peripheral circuit is needed to realize this method. Moreover, employing such a method does not solve the fundamental problem of lifetime. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.
In addition, a high voltage is necessary for a flash memory in order to inject charge into the floating gate or removing the charge, and a circuit therefore is required. Further, it takes a relatively long time to inject or remove charge, and it is not easy to increase the speed of writing or erasing data.